A liquid crystal light valve that includes a liquid crystal cell of the twisted nematic type positioned between first and second light polarizers is described in copending U.S. patent application of Buzak et al., Liquid Crystal Light Valve with Electrically Switchable Secondary Electron Collector Electrode, filed concurrently herewith. The liquid crystal cell includes a dielectric target surface positioned within an evacuated envelope, an optically transparent faceplate positioned in opposed relation to the target surface, and a liquid crystal material captured between the target surface and the faceplate.
Visible light emanating from a light source propagates through the first polarizer to illuminate the liquid crystal cell. A writing electron gun and an erasing electron gun communicate with the interior of the evacuated envelope and direct toward the target surface of the liquid crystal cell a writing beam and an erasing beam, respectively. The writing beam and the erasing beam sequentially strike preselected locations on the target surface to cause an emission of secondary electrons and thereby develop an electrostatic potential at such preselected locations. The emission of secondary electrons is accomplished in accordance with beam current modulation at the preselected locations, which define the display image.
A secondary electron collector electrode of, for example, the wire mesh type or the wire grid type is positioned over and in substantially parallel spaced-apart relation to the target surface collects in a generally uniform manner the secondary electrons emitted by all regions of the target surface. The writing beam and the erasing beam are directed through the collector electrode and toward the target surface. A collector electrode controller circuit sequentially applies first and second potential differences between the target surface and the collector electrode in synchronism with the striking of the preselected locations by the respective writing and erasing beams.
The first potential difference causes the collector electrode to collect a sufficient number of the secondary electrons to maintain the electrostatic potential at the preselected locations on the target surface. This causes the liquid crystal cell to transmit light in a first polarization sense. The second potential difference causes the collector electrode to collect a relatively small number of the secondary electrons. The remaining secondary electrons redistribute over the target surface and change the electrostatic potential at the preselected location. This causes the liquid crystal cell to transmit light in a second polarization sense. The light transmitted by the liquid crystal cell propagates through the second polarizer which transmits light in one of the first and second polarization senses to provide the display image.
Since the writing beam is directed through the collector electrode and toward the target surface, some of the writing electrons strike the wire segments of the collector electrode. These electrons are intercepted by the collector electrode and are not transmitted toward the target surface. This causes unwanted charge modulation on the target surface and results in the casting of wire shadows by the collector electrode on areas of the target surface aligned with the wire segments. The wire shadows prevent the writing beam from addressing the areas they obscure and superimpose collector electrode artifacts on the displayed image.